Extrusion

The extruder is the predominant machine of
an extrusion plant.

It allows the material to reach the required break-down and to flow through the die, which is firmly assembled to its head. Raw rubber is normally cut in stripes and feeds the internal screw of the extruder, which transfer energy to the rubber itself and makes it machinable from the die into the desired profile.
The most important parameters of temperature and internal pressure are ruled by PLC and controlled by skilled personnel.

The production lines of Tecnoextr Spa actually include extruders both horizontal than vertical, for the manufacturing of a wide range of different profiles and tubes, with diameters from a few tenth to around 130 mm.

Even if it is required just an easy machining to get them, the dies must be carefully designed and, if necessary, modified for being tuned.
First of all, extreme care and a long experience must be dedicated to designing the orifice.

The extrusion process of rubber materials avails itself of the following components:

• Extruder
• Die
• Curing oven
• Cooling bath
• Spooler
• Cutter
• Postcuring ovene

Extrusions dies for rubber are usually machined by electroerosion from steel discs, having diameter and thickness as a function of the rubber material and of the kind of profile and, in case of hollow shape, cores supported on the slotted disc with a steel frame.

A rubber uncured extruded profile flowing out from the die is free to varying its own geometry, because after being piped under a growing pressure on the screw, leaving the die the pressure drops to relative zero.
Because of this rush of pressure, which leads to swelling due to the increase of material entropy at a high temperature and low viscosity, the extrudate may collapse and vary its shape under relaxation.

Vulcanization, which implies a rapid raise of viscosity, takes place and starts in the oven, so between die and oven the extrudate doesn’t have to vary its own geometry or, at least, it has to be predicted and controlled.

All this makes clearer that quite often the geometry of the die orifice is very different if compared with the section of the desired extruded profile.

This is a typical case of reverse engineering, when one has to determine a system to producing the wished result starting from the result itself: the extruded profile according a drawing.

extruder head and locking device
It has to be evidenced that all the dies must be
developed and tuned according to the material of
the profile, which may have a certain hardness or
viscosity, and to the profile geometry.
It has to be furthermore remarked that, even if the
orifice geometry one can see on the external side
of the die may be considered as ideal and correct
for a certain profile, the internal side is usually

shaped differently and then eroded step by step in
order to get a uniform material flow in the die land.
For example, it is possible to accelerate the
material flow reducing friction losses dulling the
edges or creating converging sections. On the
contrary one can slow down the flow rate locally
by means of artificial friction losses inserting so
called brakes.

Of course, when possible, it is better to get a uniform wall thickness of the profile so that a constant mass flow rate is reached and to avoid areas thicker than the rest or edges where material stagnation or cracks may take place.
ref. 16 - isovelocity surfaces on a profile section The whole wide range of extrusion dies manufactured by Tecnoextr Spa have been developed inside the company thanks to a long experience in this field, so today die design has become a daily task and the results of this activities have been valued and called for by the market.

A few centimeters downstream from the die the profile is carried on the curing oven belt, where it crosslinks at controlled temperature according imposed parameters depending upon extruded material and profiles geometry.

Soon afterwards, if necessary the profile is cooled down in water, spooled, cut or punched.

Depending upon the material extruded, the profiles are postcured in hot air ovens according specific cycles to completing crosslinking and to completely releasing additives and vulcanization additives.
Tecnoextr Spa supplies extruded endless profiles in bundles or spooled over plastic or paperboard spools.
A collection of profiles is showed on the last pages of the catalogue.
This list, here showed partially, is continuosly updated and integrated with new articles.
Tecnoextr Spa remains at disposal of his own customers for further information and to suggesting the best fitted solution for the application required..

Tecnoextr Spa transform a wide range of rubber
materials, both compact than cellular, into
reliable and carefully manufactured extruded
products.
What it may concern about material performances
is mentioned on tables and graphs in the
introduction area of this catalogue.

Other detailed information related to our products may be requested to our technical department, which
remains always at disposal of the customers.
Extruded rubber profiles require a certain technological cycle of production, similar to what
it is used for other materials, but with a distinctive difference. What it makes the difference between rubbers and the rest of the polymers is the vulcanization process a rubber part has to withstand before its use.

This process, which implies an irreversible crosslinking of the molecules, takes place at atmospheric pressure transfering heat to the profile.

This heat transfer must be gradual and it is dependent from profile geometry, profile dimensions and polymer type.

It is evident that, even if one may try to accelerate the process of vulcanization, it starts and then it stops the shape variation only after a certain period, usually some seconds.
Anyhow it is furthermore intuitive that during this short time the profile is under an unstable equilibrium of forces and that its shape may change under its own weight because of the material relaxation.
Termoplastic extruded profiles, on the contrary, are usually calibrated and this is made possible because of the absence of vulcanization and so it is possible to cool them rapidly all over calibrators.
Tecnoextr Spa manufactures extruded profiles in conformity to severe standards with dimensional tolerances mostly according UNI ISO 3302-1.
According to the above mentioned standard norm, the suggested class of tolerances for cross-sectional dimensions of extruded profiles, except special cases, is the class E2.
Concerning products cut from tubes or cords and profiles in general, instead, we apply the class L2.

Tecnoextr Spa manufactures even extruded profiles made in different qualities of cellular rubbers. Tolerances related to such this kind of materials haven’t been included in a standard
norm. Because of the difficult prediction of the material behaviour during manufacturing, due to an enormous swelling coefficient related to the amount of blowing agents added in the compound, Tecnoextr Spa suggests for these materials the
class E3 for cross-sectional dimensions.

However the E2 class is allowed for simplified geometry like square or circular cross sections.
It is advisable to consult our technical department for further details and information.
Tecnoextr Spa plays a distinctive role on the market because of the products quality and of the technical support to the customersi.

A huge amount of different applications have been covered by alternative series of products:

• Inflatable seals, which are extruded hollow profiles joined as closed circles or as opened frames with vulcanized ends, able to expand while inflated with compressed air.
• Profiles modelled from uncured extrudate, usually silicone hoses handmade or vacuum modelled.
• Tubes and profiles with moulded ends, usually tubes with connectors or elbows moulded over to improve assembling features.
• Seal frames, manufactured from cut profiles joined by corner moulding or using specific glues.
• O-rings, joined by mouding or glued from cords or tubes.
• Flat gaskets and hoses cut from tube.

Tecnoextr Spa is certified UNI EN ISO 9002 since 1994.
Quality procedures rule the whole manufacturing process, from design to deliveries and produce
an efficient pre and post sales service and a reliable finished product marked distinctively by
competitive prices and by an excellent global quality level..

Tecnoextr Spa works at open books with its own customers. We do not hide our capabilities and our technologies and their costs, because wÉre convinced that only a sincere and cooperating relationship between us and our customers may completely satisfy them and that this honourable dealing is recognized.

With regard to that, it follows a useful scheme for an estimated profile cost calculation, trusting that it may result even helpful for a first
understanding of the argument.

Of course it would be impossible to substitute with this simple mathematical tool our technical and commercial department while estimating these costs, but wÉre sure that, even if with some limitation, these information may result appreciated and helpful.

One can get some helpful suggestion from this simplified formula.
Some materials are more expensive than others and it is not always true that a cheaper material would show worser performances.
It depends upon the application required.
Before proceeding to an inquiry evaluate carefully the whole application, not only the extruded profile and choose first a material with the best ratio between price and performance.
In case of doubts our technical department is always at your disposal.
Do not oversize unuselessly an extruded profile.
The bigger the cross section, the higher the weight per linear unit and the higher the cost due to the material. Furthermore, usually increasing
the weight per linear unit the extrusion velocity ratio lowers and so the process costs raise.

If possible adopt profiles with a constant wall thickness.

Each sensible variation of the wall thickness along the cross section implies mass flow differences.
All this afflict the extrusion velocity ratio, which has to be lowered with a consequent raise of the process cost.

An extruded profile with a complex geometry often requires the operator being constantly close to the extruder controling the process. In case of simple profiles this may be avoided, because a single operator may govern more than one extruder, with a consequent reduction of costs.
A simple slotted die for a compact profile is usually cheap.
A special die for an hollow profile is usually more expensive.

Increasing the number of cavities in a profile the die cost grows and the weight per linear unit of the profile decreases.
Try to evaluate if it is the case to invest money for tools, a multicavity die, o if it is better a higher unit weight respect to what is possible in theory.

Small batches of an extruded profile manufactured in a certain number of steps for a certain total amount of meters cost a lot more than a single batch, because of the succession of stopped sessions and repeated machine set up.
Each secondary operation has its own cost.
Verify if postcuring, cut, punching or surface treatment are necessary.
To eliminate or substitute one of these operations may result possible and consequently convenient.